2D Gaussian blurs or 1D Gaussian blurs

When deciding between a 2D gaussian blur and two 1D gaussian blur passes, it is normally best to go two 1D gaussian blur passes. The reason for this is the difference in efficiency and outcome. Both gaussian do the same function, but differently. The primary difference is what they calculate and not necessarily the exact method as they both take the same approach.

A 2D Gaussian blur takes a particular fragment coordinates and blurs in all directions set by the user. This is nice but can quickly become inefficient. Think about it this way: what if you set your blur square to 5x5? This means your blurring 5^5 pixels, for every pixel on screen. While the end result doesn't look all that bad, it has many weaknesses. 2D Gaussian's are square like by nature and can handle flat, non-circular edges but in terms of anything round, it looks far less impressive. This is where two 1D Gaussians come in handy!

2D Blur, note the blocky edges

Two 1D Gaussian do almost the same thing but instead of sampling a set of 2D coords, it samples two sets of 1D coords. This is very interesting because it is not only more efficient but also looks better. Instead of producing a box type effect, we blur a particular pixel both in the X and Y direction, but not on any sort of angle. This way, we get a nice, smooth circular effect in which looks nice and is much more efficient then the previous style.

Nice and smooth two 1D passes

In conclusion, 1D is both faster and prettier!